Noble metal paste compositions comprising novel liquid carriers



United States Patent 3,407,081 NOBLE METAL PASTE COMPOSITIONS COM-PRISING NOVEL LIQUID CARRIERS Kermit H. Ballard, Lewiston, N.Y.,assignor to E. I. du Pont de Nemours and Company, Wilmington, Del., acorporation of Delaware No Drawing. Continuation-impart of applicationSer. No. 451,037, Apr. 26, 1965. This application Sept. 20, 1967, Ser.No. 669,310

12 Claims. (Cl. 106-1) ABSTRACT OF THE DISCLOSURE The novel noble metalpaste compositions, which can be applied to green ceramic dielectricsheets in the production of capacitors, are dispersions of a particulatenoble metal(s) in a liquid carrier comprising a solution of apolyterpene resin in an aliphatic petroleum naphtha having a boilingpoint in the range of 150 to 360 C.

CROSS-REFERENCES TO RELATED APPLICATIONS This is a continuation-in-partof US. patent application Ser. No. 451,037, filed Apr. 26, 1965, nowabandoned.

BACKGROUND OF THE INVENTION Deyrup US. Patent 2,389,420 describes thepreparation of multiplate monolithic ceramic capacitors employing atechnique involving spraying a slurry of a finely divided ceramicdielectric material onto a base surface, drying the same, thensuperimposing thereon a layer of finely divided conductive metal, e.g.,silver, in paste form. The silver layer is usually applied by screenstencilling the silver paste in the desired pattern, later to serve asthe capacitor electrode or plate. As many alternate layers as desired ofceramic dielectric and silver electrodes may be built up in this manner,with the bottom-most and the upper-most layers being dielectric layers.Alternate silver layers are offset slightly so as to be exposed atopposite sides of the capacitor structure, which structure is then firedat the fusing or sintering temperature of the dielectric material toform a monolithic capacitor structure. The alternate electrode layers orplates exposed at opposite sides of the fired capacitor are thensilvered with a silver paste or paint. After again being fired, thefired silvered edge on one side connects all alternate plates exposed onthat side, while the opposite silvered edge similarly connects allalternate plates exposed on that side. Lead wires are then attached,e.g., by soldering, to

the silvered edges. Alternatively, silvering of the edges to which thealternate silver layers are exposed can be effected before the firstfiring, in which case only a single firing is necessary.

An improved modification of the method of the above patent involves theuse of thin preformed sheets, films c-r strips of green ceramicdielectric material. Such sheets are first coated, e.g., by screenstencilling, with a noble metal electrode coating in the desiredpattern, following which the sheets are stacked to provide alternatedielectric and electrode layers, with alternate electrode layers exposedon opposite edges of the stack. The stack is compressed under a pressureof about 100 to 500 psi, then fired to provide the monolithic multiplatecapacitor. The noble metal component of the electrode used should be ametal which will not melt at the firing temperature. The edges withexposed electrodes are metallized using a conductive metal paint. Thiscan be done before the stack is fired, or after such firing, dependingupon the firing temperature required and the metal paint used. If

the edges are so metallized after the stack is fired, a sec- 3,407,081Patented Oct. 22, 1968 ond firing will be required to fire suchconductive paint to the edges.

By green vitreous dielectric material is meant an unfired admixture,e.g., in a preformed shape such as a sheet, film or strip, comprising aparticulate vitreous dielectric material such as glass, barium titanate,lead zirconate and titanium dioxide, and a temporary resinous organicbinder therefor. The particulate conductive metal component of theelectrode pastes employed in forming the electrode coating is generallya noble metal such as silver, gold, palladium, platinum, rhodium,ruthenium, osmium or iridium, or mixtures or alloys of two or morethereof.

The organic binder component of the green dielectric sheet should havegood burning properties and should yield a sheet which is flexible,otherwise, cracks and other defects occur during their handling. It isalso highly important if not essential that the liquid carrier in whichthe noble metal electrode material is applied to the green dielectricsheet not be a solvent for the resinous organic binder component of thegreen dielectric sheet, otherwise, objectionable curling, cracking,and/or deformation of the electrode coated sheets result during thestacking, compressing and firing operations.

Accordingly, it is highly desirable to provide improved noble metalelectrode paste compositions especially adapted for application to greenceramic dielectric sheets in the production of fired multiplatemonolithic capacitors. More particularly, it is desirable to provideimproved noble metal electrode paste compositions which can be appliedto green ceramic dielectric sheets in the production of such capacitorswithout causing the curling, blistering, cracking or penetration of suchsheets.

SUMMARY OF THE INVENTION The improved noble metal electrode pastecompositions of the invention are dispersions of a particulate noblemetal, a mixture of two or more noble metals, an alloy of two or morenoble metals, or a mixture of two or more such alloys, in a liquidcarrier comprising a solution of a polyterpene resin in an aliphaticpetroleum naphtha having a boiling point or boiling range in the rangeto 360 C.

The noble metal pastes of this invention are useful in the production ofhighly reliable capacitors which do not possess the previously describeddisadvantagesof the prior art.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The polyterpene resins dissolvereadily in the aliphatic petroleum naphthas of the above type to producesolutions or carrier liquids which are well suited for use as carriersfor the particulate noble metal electrode materials in the applicationof the latter to green dielectric ceramic sheets. The latter generallycomprise thin flexible sheets of mixtures of a particulate ceramicdielectric material and a temporary organic resin binder which is mostgenerally a solid ethyl cellulose resin, or a solid polymer of anacrylate or methacrylate ester of a l to 4 carbon aliphatic alcohol.Such resins are favored as temporary binders for the ceramic particlesbecause they possess excellent burning properties and yield flexiblegreen sheets which handle well. These resins are insoluble in thecarrier liquid component of the present noble metal compositions.Accordingly, application of the electrode compositions of the inventionin the formation of electrode patterns onto the thin green ceramicdielectric sheets has no deleterious effect upon such sheets duringtheir handling in the stacking and compressing operations, and theresulting assembly of stacked and compressed sheets can be firedaccording to usual firing practices to burn out all' organic matter andproduce Well-formed, highly satisfactory monolithic capacitorstructures.

The solvent component of the liquid carriers for the particulate noblemetal must be an aliphatic petroleum naphtha. Aromatic hydrocarbons andthe common alcohols, ketones, esters, nitriles and the like aregenerally unsatisfactory as solvents in that they will generally exert asubstantial solvent effect upon the ethyl cellulose and the polyacrylateand polymethacrylate resins used as the temporary binders in the greensheets. The use of a watersoluble binder in the green sheet, whichbinder would be insoluble in an organic liquid carrier for the noblemetal electrode material, has not proved to be satisfactory. This isbecause effective plasticizers for such Water-soluble binders are notavailable and, without plasticization, green sheets formulated with suchwater-soluble binders undergo excessive curling and cracking duringhandling.

The liquid carrier for the particulate noble metal must include atemporary binder for the metal particles, which binder must be solublein the aliphatic petroleum naphtha solvent. The binder must also be onewhich will volatilize or burn out clearly during firing. I have foundthat the polyterpene resins having a molecular weight of from about 350to 870 are highly satisfactory binder materials. They are commerciallyavailable resins sold under the trade designation Nirez polyterpeneresins. Those having molecular weights of from 550 to 800 are preferred.

The polyterpene resin should be present in the liquid carrier employedin an amount sufficient to bind effectively the noble metal particlestogether and to the green dielectric sheet during handling. Amountsequal to to 75% of the total weight of the liquid carrier are generallyeffective; the preferred amounts are to 70%. The solvent shouldgenerally constitute essentially the remainder of the liquid carrier andshould be suflicient in amount to impart the desired fluidity to theliquid noble metal electrode composition. Insoluble particulate solidorganic or inorganic diluents, e.g., corn starch or barium titanate, maybe added in amounts to produce proper paste rheology, particularly whenan electrode composition having a relatively low noble metal content isdesired. Generally, the noble metal content of the electrode compositionwill range from to 70% by weight. The preferred metal contents rangefrom to and when these preferred amounts are employed, the use of soliddiluents such as starch is seldom required.

As indicated previously, the aliphatic petroleum naphtha used as thesolvent component of the liquid carrier should have a boiling point orboiling range (at atmospheric pressure) within the range to 360 C. Lowerboiling solvents volatilize too rapidly while higher boiling solventsvolatilize too slowly for practical use. Kerosene and diesel fuel gradeshaving boiling ranges within the above ranges are examples of suitablenaphthas. Also, any mineral oil having a boiling point within the rangeof 150 to 360 C. may be utilized. For example, a commercial mineral oilsuch as Nujol is suitable. The preferred naphthas boil within the range150 to 320 C. and most preferably within the range of to 275 C.

In use, the noble metal electrode paste composition is applied to thegreen dielectric ceramic sheet in the desired pattern, e.g., by screenstencil printing or in any other suitable manner, following which thesolvent component of the noble metal paste coating will usually beessentially completely evaporated from the coating before the coatedsheets are stacked, compressed and fired. However, in some instances thecoated sheets are stacked before the solvent is evaporated, since thepresence of the solvent assists in holding the sheets together beforeand during the compression and early firing operations.

In the following examples and elsewhere in the application, all amountsand contents expressed as parts or percentages are by weight.

The following are examples of liquid carrier compositions which aresuitable for use in formulating the noble Liquid Carrier Composition A BC D Nirez #1085 resin, percent 69 70 70 60 Kerosene (13.11. 177271 0.),percent.. 31

Aliphztie petroleum naphtha (B.R. 1

con

Aliphtatic petroleum naphtha cen Diesel fuel #208 (B.R. 204-315 0.),percent 40 The above compositions were prepared by sifting thepulverized resin into the solvent While the latter was being stirred,then continuing the stirring at 3952 C. until the resin was completelydissolved.

The solvents employed in the above compositions varied considerably asto their rates of drying or evaporating. Thus, when 10 gram portions ofthe solvents used in compositions A, B, C and D were placed on watchglasses (5 inches in diameter) at room temperature in still air, theproportions evaporated in 64 hours were 75, 70, 43 and 20% respectively.

' The following silver electrode paste compositions were prepared bymixing appropriate proportions of powdered silver and one of the abovecarrier liquids in a 3-rol1 paint mill to effect thorough dispersion ofthe silver in the carrier liquid. If the viscosity of the resultingpaste is somewhat higher than desired, it can be lowered as desired byadding suitable amounts of the appropriate naphtha solvent. These pastecompositions are suitable for use when the green ceramic sheet to whichthey are to be fired is formulated using a low firing ceramic dielectricsuch as a glass.

SILVER ELECTRODE PASTE COMPOSITIONS FOR LOW FIRING CERAMIC DIELECTRICSWeight Percent Example l 2 3 4 Silver powder Carrier liquid A... Carrierliquid B.

Carrier liquid D The following are examples of noble metal electrodepaste compositions suitable for use when the green sheet is formulatedof a high firing ceramic dielectric.

NOBLE METAL ELECTRODE PASTE COMPOSITIONS FOR HIGH FIRING CERAMICDIELECTRICS Weight Percent Example 5 6 7 8 Rhodium powder. Palladiumpowder.-. Platinum powder- Gold powder Carrier liquid A..... Carrierliquid B... Carrier liquid 0. Carrier liquid D...

Example 9 A flexible self-supporting green sheet of a low firing glassdielectric is prepared as follows: a glass frit is made by meltingtogether 45.4 parts PbO, 23.6 parts SiO 3.9 parts K CO 3.9 parts NaF,7.5 parts MgCO' 1.6 parts Li CO 2.4 parts Na CO and 11.7 parts Sr(NOuntila clear fluid melt is'obtained,'which is fritted by pouring themelt into water. The resulting coarse frit is ball milled for 16 hoursin a porcelain mill with porcelain balls and about /3 its weight ofwater, filtered, dried and crushed to a powder having an averageparticle size of about 5 microns. This frit is slurried in such anamount of a solution of ethyl cellulose (200 cps.) in acetone as willprovide an amount of ethyl cellulose equal to 10% of the weight of thefrit. The slurry is cast on a clean glass slab which had been previouslycoated with dilorol phosphate, a stripping agent, using a doctor bladethat gives a film 5 mils thick. After drying and stripping from theglass slab, the flexible green sheet will have a thickness of about 4mils.

Using the screen stencil technique, /2" x 1" electrode patterns of thesilver electrode paste of Example 1 are printed upon several sections ofthe above green dielctric sheet. After drying, the printed sheets(sections) are stacked so that about A" of the patterns on'successivesheets overlap, with about /E;" of thepattern of alternate sheetsextending to-the right beyond the overlap area, and with about Ma" ofthe pattern of the remaining sheets extending to the left beyond theoverlap area. After 8 (or any desired number) printed sheets have beenstacked or built up in this manner with an unprinted top or cover sheet,the stack is carefully compressed under a pressure of about 300 p.s.i.,then die cut so as to expose the electrode prints of alternate sheets onopposite ends of the stack. The opposite ends with the exposed alternateelectrode prints are painted with a silver paint and the stack is thenslowly fired on an ethyl cellulose coated stainless steel plate fromroom temperature to 760 C. over a period of 16 hours.

The resulting monolithic capacitor structure consists of 8 electrodes,each having a plate area of about /2 x A", with each being separatedfrom each other by a ceramic layer about 4 mils thick having adielectric constant of about 10. Except for the use of the greendielectric in preformed sheet form, this method of forming themonolithic capacitor structure is essentially that described in DeyrupUS. Patent 2,389,420.

In forming monolithic capacitor structure as described above, the silverelectrode composition does not attack, blister or perforate the greendielectric sheet during its application thereto or during subsequentstacking, com'- pressing and firing operations. Consequently, nocurling, cracking or other related difiiculty is experienced in theentire operations by which the fired monolithic structure is obtained.

Example 10 A green dielectric sheet is prepared essentially as describedin Example 9, except that the solution in which the glass frit isslurried for casting is an 8.1% solution of a methyl methacrylatepolymer (about 65,000 molecular weight) in a solvent consisting of 40%methyl ethyl ketone, 29.3% butanol, 29.3% xylene and 1.4% dibutylphthalate. Sections of the green sheet are printed as described inExample 1 but using the gold electrode paste of Example 8. The printedsections are than processed as described in Example 9 to give amonolithic multiplate capacitor structure. No curling, blistering orrelated problems are encountered.

Example 11 A flexible self-supporting green dielectric sheet of a highfiring dielectric material is prepared following the general procedureof Example 9. In this instance, however, the dielectric material is amixture of 112.8 parts BaZrO 31.2 parts MgZrO 50 parts BaCO 12 partskaolin, 0.6 part Mn0 and 993.7 parts BaTiO which mixture is slurried forcasting in 1560 parts of the methyl methacrylate polymer solutiondescribed in Example 10. Electrode prints are applied to sections of theresulting green sheet, and the sections are then stacked, compressed andfired as described in Example 9. In this instance, however, the metalpaste used in printing the electrodes consists of 60% platinum black and40% of a carrier liquid consisting of a 40% solution of Nirez #1135resin, a polypinene resin of molecular weight of about 870, in dieselfuel having a boiling range of 204 to 315 C. Also, firing of the stackedassembly is effected very slowly over a 4 day period from roomtemperature to 1315 C. The opposite ends of the monolithic structurewith their exposed alternate electrode plates are then printed with aconductive silver paint containing a bismuth oxide-cadmium borateceramic binder, and the silver paintings are fired on at 760 C.

The area of each electrode plate of a capacitor formed as describedabove is /2" x /3" and the plates are separated from each other byceramic layers having a dielectric constant of about 7000. No curling,cracking, blistering or attack of the green sheet by the platinumelectrode composition is encountered during formation of the capacitor,and the latter is free fromfiaws, perforations and delaminations such asoccur when using a metal electrode composition containing a componentwhich is a solvent for the binder component of the green sheet.

Example 12 A monolithicmultiplate capacitor is prepared as described inExample 11, except that the metal electrode paste employed consists of60% rhodium powder and 40% of a carrier liquid consisting of a 40%solution of Nirez #1135 resin in diesel fuel having a boiling range of204 to 315 C. Results essentially similar to those described in Example11 are obtained.

Example 13 A monolithic capacitor is prepared as generally described, inExample 10 except that in preparing the green ceramic sheet, there isused a slurry of the barium titanate mixture in an 18.5% solution of an-butyl methacrylate polymer (about 300,000 molecular Weight), in asolvent mixture consisting of 97% amyl acetate and 3% dibutyl phthalate.The slurry is prepared using an amount of the polymer solution to give acast sheet containing 25% of the polymer and of the ceramic dielectricmaterial. Also, the electrode prints are made using a paste consistingof 50% palladium powder and 50% of a carrier liquid consisting of a 60%solution of Nirez #1100 resin (a polypinene resin of molecular weight ofabout 770) in an aliphatic petroleum naphtha having a distillation rangeof 200 to 238 C. A satisfactory monolithic capacitor structure isobtained without objectionable curling, cracking or blisteringoccurring.

Example 14 No satisfactory fired monolithic capacitor can be produced bythe procedure of Example 11 when that procedure is modified to employ inplace of the platinum electrode paste of that example, one consisting of60% platinum black and 40% of a carrier liquid consisting of a 60%solution of hydrogenated rosin in beta-terpineol. This carrier liquidfor the platinum exerts a solvent action upon the methyl methacrylatepolymer binder of the green sheet. As a consequence, it penetrates andsoftens the green sheets so that during the printing of the electrodesthereon they buckle and distort badly. Furthermore, during the pressingof the stacked sheets together, large sections of inner sheets aresqueezed out of the stack. Similar difficulties are experienced whenusing noble electrode paste formulated using other carrier liquids whichexert a solvent action on the temporary binder of the green ceramicdielectric sheet.

The noble metal components of the electrode pastes of the above exampleswere of particle sizes in the range 0.1 to 5 microns, and the ceramicdielectric materials of the green ceramic dielectric sheets were ofparticle sizes in the range 1 to 20 microns. In general, the noble metalcomponents of such electrode pastes should be employed in finely dividedpowder form with a particle size not exceeding about 50 microns andpreferably being in the range 0.1 to microns. The particle size of theceramic dielectric materials employed in formulating the greendielectric sheet, film or strip, will generally range from 0.1 to 50microns and preferably will be in the range 1 to microns.

Since it is obvious that many changes and modifications can be made inthe above-described details without departing from the nature and spiritof the invention, it is to be understood that the invention is not to belimited to said details except as set forth in the appended claims.

I claim:

1. A noble metal paste composition comprising a to dispersion of a noblemetal powder in a liquid carrier comprising a 20 to solution of apolyterpene resin having a molecular weight of 350 to 870, in analiphatic petroleum naphtha boiling in the range of to 360 C.

2. A noble metal paste according to claim 1 wherein the polyterpeneresin is a polypinene resin.

3. A noble metal paste composition comprising a 45 to 70% dispersion ofa noble metal powder in a liquid carrier comprising a 20 to 75% solutionof a polyterpene resin having a molecular weight of 350 to 870, in analiphatic petroleum naphtha boiling in the range 150 to 320 C.

4. A noble metal paste composition comprising a 45 to 70% dispersion ofa noble metal powder in a liquid carrier comprising a 20 to 75 solutionof a polypinene resin having a molecular weight of 350 to 870, in analiphatic petroleum naphtha boiling in the range 150 to 320 C.

5. A paste composition according to claim 4 wherein the liquid carriercomprises a 30 to 70% solution of a polypinene resin having a molecularweight of 550 to 800, in an aliphatic petroleum naphtha boiling in therange of to 275 C.

6. A paste composition according to claim 4 wherein the polypinene resinhas a molecular weight of about 5 80.

7. A paste composition according to claim 4 wherein the polypinene resinhas a molecular weight of about 770.

3. A paste composition according to claim .4 wherein the noble metal issilver.

9. A paste composition according to claim 4 wherein the noble metal isplatinum.

10. A paste composition according to claim 4 wherein the noble metal isgold.

11. A paste composition according to claim 4 wherein the noble metal ispalladium.

12. A paste composition according to claim 4 wherein the noble metal isrhodium.

References Cited UNITED STATES PATENTS 2,280,135 4/1942 Ward 252-514 XR3,158,503 11/1964 Young 117-227 XR 3,293,501 12/1966 Martin 2525l4 XRJAMES A. SEIDLECK, Primary Examiner.

L. B. HAYES, Assistant Examiner.

